When I was young in the Willamette Valley of western Oregon, the farmers would work after harvest and plant winter cover crops such as turnips/ radishes, buckwheat, or winter wheat to stop winter erosion in an area that got approximately 45 in. precipitation and to utilize the green manure in the spring, if they had access to cow, horse or chicken manure they would spread it on in the spring also, before working the ground and planting their crops. The ground was some of the most fertile ground that you could ask for. A common joke was that you threw out the seed and then jumped back so that you wouldn't get bowled over by the plants. Other common practices were crop rotation and contour plowing. Yes! It was labor intensive, BUT IT WORKED, and their ground stayed fertile and IN PLACE.
I moved to S. W. Idaho after my stint in the Navy and found a totally different type of farming. Farmers worked their fields as quickly as they could after harvest and then let the fields lay fallow for the winter. When spring came, they would spread prelled fertilizer on the fields. Then when the wind would blow and hey would loose topsoil, fertilizer and some times seed. It was not unusual for neighbors to have different plants than what they planted and for there be auto wrecks because of the lack of visibility.
My farming in this valley has primarily been alfalfa seed, hay and pasture. When I first moved here i was told that I had to use a cover crop when I planted. Over the last 40 years of experimenting and talking to farmers all over the west, i came to the conclusion that cover crops are and asset in some cases, they should absolutely not be used in other cases and the rest of the times that there was no benefit. What worked the best was to plant a winter green manure crop in the fall and in the spring disk and groundhog it in just before planting your crop. What I found worked was to disk it with a groundhog attached to the disk. Depending on the soil and growth conditions you may have to go over the field twice. This leaves enough vegetative matter in the soil to keep it from blowing away. It will also leave moisture in the top several inches of the soil to help germinate the seed that you drill in. After 4 or 5 years I noticed that i didn't need nearly as much fertilizer. Where I was putting down composted manure, I found that i didn't need any fertilizer. One of my friends has been doing this for years. He has a field that he has planted to corn for the last 14 years. His yields off of that field are higher than any of the neighbors and he hasn't put any nitrogen down for the last 10 years.
There are a lot of ideas for what makes a good winter green manure. As a kid we used buckwheat or turnips. Many a morning I was sitting on our green manure fields during deer season. One of the nice things about the right green manure fields is that they can be winter grazed some. A mixture that I like is: 1) Buckwheat 2) Austrian peas 3) Turnips / Radishes
4) Millet Buckwheat puts phosphate into the soil, Austrian peas puts nitrogen into the soil, Turnips / Radishes helps build humas and allows moisture to sub better and millet gives good cover and provides extra grazing. (NOTE: don't graze dairy cows on this unless you really like turnip flavored milk) I know that there are many other plants that can be used and some are even necessary for certain soil conditions. We will mix any combination that is desired. My grandpa used to tell me "Take care of your land and it will take care of you" I've always found that to be good advise.